Ethylene and Auxin Participation in Pollen Induced Fading of Vanda Orchid Blossoms

Blossoms fromii several varieties of orchids fade prematurely when their pollinia are removed or disturbed (2,3,7), and also if the flowers are gassed with ethylene (8, 9, 10), pollinated (3, /7), or treated with an auxin (3, 7). During natural Ifading as well as that induced by removal of the pollinia, ethylene is evolved (2, 3, 9, 10), andtherefore it has ibeen inferred that the gas is the catusative agent. Ethylene evolution also is stimlulated Nvllen plant tissuies are exposed to auXin ( 1, 4), andl wve now )resent evidence that this response is the basis for flower fading1 indtuced by pollination or an,xin application. Blossoms from ['undo Petanboeran or Van(la Rose .M1arie were iplaced wvitlh their cut ends in a beaker of water, and( sealed nudtler a 200 nlm bell jar having a ground glass base anid a side arm closed with a rubber vaccine cap. Otlher flowers svere pollinated, emilasculated (pollinia removed), self-pollinated, or treated with 5 mnm IAA in lanolin or 0.1 mm carboxyl lajbeled '-C-IAA in 0.8 % agar (specific activity 8 mc/mmole) applied to the stigmas, and then the flowers were sealed under a bell jar. Air samples, removed with a hlypodernmic syringe, were analyzed by gas chromatography to determine the ethylene content (4), anid at least every 10 hours the chambers were completely aerated. Ethylene evolution by isolated sepals and petals, columns or lips was measured by incubating each tissue in a 50 ml flask fitted with a vaccine cap. and sampling the air in the flask at hourly intervals. Cutting the floral parts induced a wound response, lasting about 1 hour, during whichi time 1.5 mkdJ of ethylene per gram of cut control tissue 'was produced. Treated cut tissue was only considered to produce ethylene if the total evolution exceeded this value, and if the extra evolution contintued at a nearlv constant rate for at least 4 hours. Intact blossoms were also placed in desiccators and gassed for 3 to 24 hours with 10 ppnm ethylene. In suich cases the flowNers were aerated f'or at least 1 hour 'before ethy;lenle production was measured in order to remiove any ethylene contained within the tissue. The sipread of 1AC-IAA from the stigmas of the flower was determined by excising the 'floral parts from representative blossomiis after 3, 6, and 24 hours and dividing these into pieces as illustrated in figure 3. Each piece was ground with ethanol on a planchet, dried, and counted w ith a 21 % efficient gas flow counter. When blossoms of V. Rose Marie were emasculated, ethylene evolution began after a 10 hour lag period (fig 1), and fading became obvious after an additional. 8 to 12 hours. A similar time course of ethvlene production and fadintg hias been reported for V. Miss Agnes Joaquitin blossoms after renoval of tlieir pollinia (2). whereas F. Petarnioeran, a semiterete of heav texture, fades after a considerablv longer lag(fig 2). That remiioval of the pollinia per se is not responsible for floral fading is indicated bv experiments with Phalcanopsis Pamiiala (7) which have showun that disturbance of the connection between the rostelluii (tthe structure supporting the pollinia) and the upper surface of the sticky disk with which it is in intimate contact, suffices to induce the response. There is no indication that this tvpe of fading ,in.volves a release or production of auxin, for it does not result in swelling of the column whereas both auxin application and pollination bring about such an effect. Self ,pollination induces ethylene formation within 1 hour in V. Rose Marie, and flower fading within 8 to 10 bhours (ifig 1). A simil.ar time course was observed with V. Petamboerant even when the flower was pollinated with its own pollinia intact (fig 2). This response was duplicated by applying enough IAA (5 mm) in lanolin paste to fill the stigmatic cavity (figo2), and exactlv the same result was obtained using 0.1 bM '*C-IAA in 0.8 % agar. Other cases .n which the fading response caused by pollination is stimulated by auxin have been reported (3, 7), and led to the suggestion that the large quantity df auxin contained in the pollen is responsible for the pollination effect (7). As auxin stimulates ethylene evolution in these blossoms and other plant tissues within an hour, an(d since ethylene causes floral fading, it is logical to conclude that this is the mlechanismll involved. Auxin induced ethylene formation must also influence the growvth of the column for within 2 hours after application of pollinia or auxin this structure